We have been studying expression of 8.4 kb endogenous MCF related RNA which is found in thymuses of autoimmune-prone but not other mouse strains. The lupus-associated expression is limited to the Mpmv sub-set of MCF endogenous retroviral sequences. In contrast, the Pmv subset expression is associated with mitogenic stimulation. CD 5+ B cells, thought to be important in autoantibody secretion, are present in increase percentages in the peritoneal cavities of mice of many strains. Using in situ hybridization, we have found that peritoneal B cells produce levels of Ig mRNA intermediate between those of resting B cells and plasma cells. However, the mRNA was of the membrane rather than the secreted form. Moreover splenic B cells produced much more Ig in short-term culture than did peritoneal B cells. Thus, cells may be stimulated in the peritoneal cavity but appear to leave prior to Ig secretion.